How long can an American green card return to China?

Don't exceed 183 days at most, or your green card may be cancelled.

I usually stay in China for three months and then go back.

If you are in a hurry, you can go back to China for three months, then go back to the United States for one week, and then come to China for three months, which is relatively better. If the immigration department asks you after the emergency, you have reason to explain.